Field of the Invention
The invention lies in the electronics and circuit technology fields. More specifically, the invention relates to a configuration with a current source and a switch connected in series with the current source.
A configuration of the generic type is illustrated in FIG. 2. The circuit of FIG. 2 contains four transistors P1, P2, N1 and N2 connected in series, and
the first transistor P1 is a PMOS transistor, whose source terminal is connected to the positive pole VDD of a supply voltage supplying the configuration with power, and which is controlled by a signal icp_refp;
the second transistor P2 is a PMOS transistor, whose source terminal is connected to the drain terminal of the first transistor P1, and which is controlled by a signal upq;
the third transistor N1 is an NMOS transistor, whose drain terminal is connected to the drain terminal of the second transistor P2, and which is controlled by a signal down; and
the fourth transistor N2 is an NMOS transistor, whose drain terminal is connected to the source terminal of the third transistor N1, whose source terminal is connected to the negative pole VSS of a supply voltage supplying the configuration with power, and which is controlled by a signal ipc_refn.
The signal upq controlling the transistor P2 is the output signal from an inverter INV1 which is formed by a PMOS is transistor P3 and an NMOS transistor N3 and which inverts a signal incr fed to it.
The signal down controlling the transistor N1 is the output signal from an inverter INV2 which is formed by a PMOS transistor P4 and an NMOS transistor N4 and which inverts the output signal fed to it by an inverter INV3 which is formed by a PMOS transistor P5 and an NMOS transistor N5 and which, for its part, inverts a signal decr fed to it.
The transistors P1 and N2 are driven by the signals icp_refp and icp_refn controlling them in such a way that they respectively form a current source, the currents output by these current sources being adjustable to the respectively desired values by way of the signals icp_refp and icp_refn controlling the transistors. For better clarity, the transistors P1 and N2 are also designated below as current sources P1 and N2.
The transistors P2 and N1 are driven by the signals upq and down (incr and decr) controlling them in such a way that they respectively form a switch. It is thereby possible for these switches to be opened and closed as a function of the signals upq and down, respectively. For better understanding, the transistors P2 and N1 are also referred to below as switches P2 and N1.
The configuration has an output terminal O, which is connected to a point lying between the switches P2 and N1 and via which an output signal icp is output.
From the above-described construction of the configuration, it becomes clear that the current generated by the current source P1, or the current generated by the current source N2, or no current is optionally output via the output terminal O. Stated more precisely:
the current generated by the current source P1 is output if and as long as the switch P2 is closed (i.e., the transistor P2 forming the switch is turned on);
the current generated by the current source N2 is output if and as long as the switch N1 is closed (i.e., the transistor N1 forming the switch is turned on); and
no current is output if both switches P2 and N1 are open (the transistors P2 and N1 forming the switches are off).
The configuration shown in FIG. 2 is a current source which can be used universally. It is possible, with that current source, to output a current of any desired magnitude for any desired period and at any desired times.
However, this is true only in theory. In practice, problems can occur which restrict the possible uses of the configuration. These problems are that, from time to time, following the closure of the switches P2 and N1, a current is output for a certain time which is higher or lower than the current actually to be output (than the current output by the current sources P1 or N2); experience shows that the current output after the closure of the switches can, for a certain time, be higher or lower by up to several hundred mA than the current actually to be output.
Inter alia, this results in the configuration shown in FIG. 2 not being usable
if very short current pulses of defined magnitude are needed, for example if a current of 10 mA is needed for a duration of 0.5 ns or less; and/or
if (irrespective of the duration during which the configuration outputs a current) larger deviations of the current output by the configuration from the current actually to be output are impermissible.
It is accordingly an object of the invention to provide a current source and switch configuration, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and wherein the current output by the configuration is always as large as desired, in particular also immediately after the closure of the switch.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration, comprising:
a current source and a current switch connected in series with the current source, the current switch having a current-source side terminal;
a control device configured to ensure that a potential established at the current-source side terminal of the current switch, when the current switch is open, has a value equal to a value the potential would have if the current switch were closed, under otherwise unchanged conditions, and if a current output by the current source were to flow through the current switch.
In other words, the configuration according to the invention is defined by the fact that it contains a control device which ensures that the potential established on the current-source side terminal of the switch, when the latter is open, has the value which it would have if the switch were closed, under otherwise unchanged conditions, and if the current output by the current source were to flow through the switch.
This rules out the situation where the potential established on the current-source side terminal of the switch rises in phases wherein the switch is open. Preventing the potential rise eliminates the cause responsible for an increased current flowing when the switch is closed.
In the case of conventional configurations of the type of FIG. 2, during phases wherein the switch is open, a potential rise inevitably occurs on the current-source side terminal of the switch. The reason for this is that the current source also outputs a current after the switch has been opened. The current which continues to flow results in an increased amount of charge accumulating in the section of line running between the current source and the switch, and this in turn results in the potential established there rising. The increased potential, more precisely the increased amount of charge causing this potential increase, has the effect that when the switch is closed, not only does the current output by the current source flow but, in addition, an additional current resulting from the decay of the increased amount of charge, the speed at which the additional current decays depending on the capacitance of the section of line running between the current source and the switch.
The fact that, in the configuration according to the invention, the potential that is established on the current-source side terminal of the switch is brought to a specific value and/or kept at a specific value means that no increased amount of charge can accumulate in the section of line running between the current source and the switch, and, consequently, no additional current can flow either when the switch is closed.
The setting, carried out described, of the potential that is established on the current-source side terminal of the switch means that the conditions are satisfied which must be satisfied in order that the current that flows through the switch when it is closed is exactly the current output by the current source and, consequently, the current output from the configuration is exactly the current output by the current source. Because this condition is satisfied at all times, that is to say including the time of closing the switch (whenever this takes place), the current that flows through the switch from the time the latter is closed is precisely the current output by the current source, so that the current output from the configuration is always precisely the current output by the current source, that is to say even immediately after the closure of the switch.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
In accordance with an added feature of the invention, the control device includes a control switch corresponding to the current switch, the control switch having a first terminal connected to the current-source side terminal of the current switch, and the control switch of the control device is connected and wired such that, when the control switch is closed, a current flowing therethrough equals a current flowing through the current switch when the switch is closed.
In accordance with an additional feature of the invention, the current flowing through the control switch of the control device, when the control switch is closed, is the current output by the current source.
In accordance with another feature of the invention, the current switch provided outside the control device and the control switch of the control device are driven such that the control switch of the control device is open when the current switch is closed, and that the control switch of the control device is closed when the current switch is open.
In accordance with a further feature of the invention, a potential established at a terminal of the control switch opposite of and not connected to the current-source side terminal of the current switch corresponds to a potential established at a terminal of the current switch not connected to the current source.
In accordance with again an added feature of the invention, a voltage follower is connected to set the potential established on the terminal of the control switch that is not connected to the current-source side terminal of the current switch.
In accordance with again an additional feature of the invention, the voltage follower has an input terminal connected to the terminal of the control switch that is not connected to the current-source side terminal of the current switch, and an output terminal connected to the terminal of the control switch that is not connected to the current-source side terminal of the current switch.
In accordance with again another feature of the invention, the voltage follower has first and second transistors connected in series, the first transistor is a transistor arranged in a source follower circuit, and the second transistor is used as a diode.
In accordance with again a further feature of the invention, the first transistor has a gate terminal forming an input terminal of the voltage follower, and the second transistor has a drain terminal forming an output terminal of the voltage follower.
In accordance with a concomitant feature of the invention, the current switch and the control switch are each formed by a transistor.
Although the invention is illustrated and described herein as embodied in a configuration having a current source and a switch connected in series therewith, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.